Purpose:
To determine the effect of various hydration media (PBS, BSS, and 15% Dextran) on ex vivo corneal elasticity.

Methods:
Experiments were conducted on fifteen porcine eyes (5 eyes each for PBS, BSS, and 15% Dextran; <3 days postmortem). The eyes were retrieved from an abattoir, placed in a bag filled with saline, and shipped to the laboratory overnight. Upon arrival, the epithelium was removed using a cotton swab, and the cornea was excised with a generous scleral rim and placed in 20% Dextran overnight to restore the cornea to physiological levels (average: 500µm). With the thickness restored, the cornea was mounted onto a custom chamber and immersed in a hydration medium (PBS, BSS, or 15% Dextran) for elasticity testing. While maintained in each medium, corneal elasticity measurements were performed for 2 hours; measurements were conducted at 5-minute intervals for the first 30 minutes and then at 15-minute intervals for the remaining 90 minutes. A custom-built Atomic Force Microscope designed for the mechanical testing of ophthalmic tissues was used to perform the elasticity testing. Tips of traditional AFM cantilevers modified with glass microbeads (diameter: 60-75µm) were used to ensure tissue-level mechanical response. Young’s modulus was calculated using the Hertz model for a spherical indenter. Corneal thickness measurements were taken with a pachymeter before and after elasticity testing.

Results:
The hydration medium used affects the stability of corneal thickness and elasticity measurements over time. After 2 hours, the corneas in PBS and BSS had swelled by 81.6% and 75.7%, respectively; the thickness of corneas in 15% Dextran decreased by 14%. On average, Young’s modulus of elasticity increased 168kPa, 103kPa, and 29kPa with time for corneas maintained in PBS, BSS, and 15% Dextran, respectively. There appears to be a relationship between the corneal swelling and the resultant elasticity; greater edema resulted in higher values of the Young’s modulus (stiffer cornea). As the cornea swells, the presence of the limbus is hypothesized to prevent peripheral diffusion for water escape, creating a stiffening effect.

Conclusions:
The maintenance of corneal hydration is important for stable and accurate characterization of corneal biomechanics over time. 15% Dextran is suggested to be an effective hydration media in maintaining both corneal thickness and elasticity over time.